The present invention relates to semiconductor laser devices, and in particular, though not exclusively, to a single mode index guided laser diode.
In many applications there is a desire for semiconductor laser devices to operate with a single spatial mode output. This output is desirable, for example, for increased coupling to single mode fibres, and for generating small spot sizes with high light intensities. Typically laser diodes generating single mode outputs use index guided laser structures which have either a ridge or a buried heterostructure waveguide. Such devices comprise, for instance as disclosed in EP 0 475 330, a laser structure comprising a substrate, lower and upper charge carrier confining layers on said substrate, a ridge extending over a portion of said upper confining layer and laterally confining the optical mode of said laser, whereby a layer of active lasing material is sandwiched between said confining layers, said layer comprising a Quantum Well structure and being configured as an active region.
Though these devices provide a single spatial mode output, the total output power is limited due to the Catastrophic Optical Mirror Damage (COMD) level at ends (facets) of the device. Each laser facet is cleaved semiconductor, and as such contains a high density of vacancies and broken bonds which can lead to the absorption of generated light. Light or electrical current absorbed at the laser facets generates heat as excited carriers recombine non-radiactively. This heat reduces the semiconductor band-gap energy, leading to an increase in absorption inducing thermal runaway which results in COMD.
Other problems with these devices include the propagation of higher order modes at high drive currents. These higher order modes propagate due to high levels of injected carriers influencing the refractive index and optical gain in areas immediately adjacent the active region.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a laser device (such as a single mode index guided laser) which obviates or mitigates at least one of the aforementioned disadvantages of the prior art.
It is a further object of at least one embodiment of at least one aspect of the present invention to provide a semiconductor laser device, which, by use of a diffractive region at an end of a laser region (such as a single mode index confined semiconductor laser region), provide as single mode output at increased output power levels.
It is a still further object of at least one embodiment of at least one aspect of the present invention to provide a semiconductor laser device wherein, by incorporating a passive region formed by Quantum Well Intermixing using an impurity free technique in a gain region, beam steering characteristics of the laser device can be improved, ie by reducing the tendency to beam steer.
According to a first aspect of the present invention, there is provided a semiconductor laser device comprising:
an optical waveguide;
at least one electrical contact extending along part of a length of the waveguide; and wherein the at least one electrical contact is shorter than the optical waveguide.
Preferably, at least one end of the/each electrical contact is spaced from a respective end of the optical waveguide.
In one embodiment, the optical waveguide is a ridge waveguide, and the at least one electrical contact is provided on the ridge waveguide.
By this arrangement a part or parts of the ridge waveguide will not be electrically pumped, in use. It has been surprisingly found by this arrangement, that the semiconductor laser device may be operated as a mode control discriminator/stabiliser. Since the waveguide is single mode, the non pumped portion of the waveguide with no current injection should remain single mode, in use.
Preferably a length of the optical waveguide may be around 200 to 2000 xcexcm, while a length/total length of the electrical contact(s) may be around 100 to 1900 xcexcm.
In a modified embodiment there may be provided a first compositionally disordered or Quantum Well intermixed material bounding sides of the optical waveguide.
According to a second aspect of the present invention, there is provided a method of fabricating a semiconductor laser device comprising the steps of:
(i) forming an optical waveguide;
(ii) forming at least one electrical contact along part of a length of the waveguide, such that the at least one electrical contact is shorter than the optical waveguide.
According to a third aspect of the present invention there is provided a semiconductor laser device, comprising:
an optical active region including an optical waveguide and an optically passive region(s) provided at one or more ends of the optical waveguide; wherein
the at least one of the optically passive region(s) is broader than the optical waveguide so that, in use, an optical output of the optical waveguide diffracts upon traversing the at least one optically passive region.
In this way, the optical output may be expanded so that an intensity of optical radiation (light) impinging on an output facet of the device is reduced. Hence an output power of the device can be increased without reaching the COMD limit of the output facet.
Preferably the optically active and passive region(s) are provided within a core or guiding layer between first (lower) and second (upper) optical cladding/charge carrier confining layers, which guiding layer may comprise an active lasing material.
Preferably a ridge is formed in at least the second cladding layer and extends longitudinally from a first end of the device to a position between the first end and a second end of the device.
Additionally the active lasing material layer may include a Quantum Well (QW) structure.
Preferably the optically passive region(s) may comprise a first compositionally disordered or Quantum Well Intermixed (QWI) semiconductor (lasing) material region provided from or adjacent to the aforesaid position to the second end of the device.
In a modification of the device there may be provided second compositionally disordered (lasing) material regions laterally bounding the optical active region.
The first and second QWI materials may have a larger band-gap than the active region. The first and second compositionally disordered lasing materials may therefore have a lower optical absorption than the active region.
Preferably the device may be of a monolithic construction.
More preferably the device may include a substrate layer upon which may be provided the first cladding layer, core layer, and second cladding layer respectively.
Preferably the second end or facet may comprise an output of the semiconductor laser device. The first QWI material may therefore act as a diffractive region at the said output of the laser device. The diffractive region may act, in use, to reduce the intensity of optical radiation impinging on the said facet by spreading out the optical radiation.
More preferably the facet includes an anti-reflective coating on cleaved semiconductor. Preferably the anti-reflective coating may be around 1%-10% reflective. The combination of the first QWI diffractive region and the anti-reflective coating provides a Non-Absorbing Mirror (NAM) which further raises the COMD level of the facet and consequently the output power of the laser device can be raised.
Advantageously, the first and second compositionally disordered materials may be substantially the same.
The QWI washes out the Quantum Well confinement of the wells within the semiconductor laser material. More preferably, the QWI may be substantially impurity free. The QWI regions may be xe2x80x9cblue-shiftedxe2x80x9d, that is, typically greater than 20-30 meV, and more typically, a 100 meV or more difference exists between the optical active region when pumped with carriers and the QWI passive regions. The first compositionally disordered lasing material therefore acts as a spatial mode filter as higher order modes will experience greater diffraction losses as they propagate through the first compositionally disordered lasing material than the fundamental mode. Thus the fundamental mode will have a greater overlap with the active region and be selectively amplified. The semiconductor laser device may therefore be adapted to provide a substantially single mode output.
Preferably the semiconductor laser device further comprises respective layers of contact material contacting an (upper) surface of the ridge and a (lower) surface of the lower cladding layer. Alternatively and preferably, the contact material may contact an upper surface of the ridge and a lower surface of the substrate. The contact layers may provide for drive current to the optical active or xe2x80x9cgainxe2x80x9d region. It will be appreciated that references to xe2x80x9cupperxe2x80x9d and xe2x80x9clowerxe2x80x9d are used herein for ease of reference, and that in use, the device may be oriented in any of various dispositions.
In an embodiment of the present invention the material contacting an upper surface of the ridge may have a smaller area than the area of the upper surface of the ridge. In this embodiment a contact-free portion of the ridge exists. This contact-free portion may provide a second passive region within the core layer of the semiconductor laser device. The second passive region may have a larger band-gap energy and therefore lower absorption than the active region. The second passive region may be formed by Quantum Well Intermixing as hereinbefore described.
Preferably the second passive region may be part of the ridge. Preferably also an end of the second passive region is provided at the aforesaid position such that the second passive region is at an effective xe2x80x9coutput endxe2x80x9d of the laser device. The second passive region may assist in correct beam steering.
Preferably, a length from the position to the second end of the device may be around three orders of magnitude smaller than a length between the first and second ends of the device. Preferably also the second passive region may be substantially smaller in length than the passive region, i.e. the distance between the aforesaid position and the second end of the device.
In an embodiment of the semiconductor laser device, the semiconductor laser device may have a ridge width of around 1 to 5 xcexcm, a width of around at least three times that of the ridge, and preferably around 15 xcexcm, a distance between the ends of around 1-2 mm, a distance between the first end and the position of around 1.5 mm, and a passive region having a length of around 0.5 mm.
Preferably the semiconductor laser device is fabricated in a III-V materials system such as Gallium Arsenide (GaAs) or as Aluminium Gallium Arsenide (AlGaAs), and may therefore lase at a wavelength of substantially between 600 and 1300 nm. The first and second compositionally disordered materials may substantially comprise Indium Gallium Arsenide (InGaAs). It will, however, be appreciated that other material systems may be employed, eg Indium Phosphide, (InP), and may therefore lase at a wavelength of substantially between 1200 and 1700 nm.
According to a fourth aspect of the present invention there is provided a method for fabricating a semiconductor laser device comprising the steps of:
(i) forming in order:
a first optical cladding/charge carrier confining layer;
a core (lasing material) layer, in which is formed a Quantum Well structure; and
second optical cladding/charge carrier confining layer;
(ii) forming a passive region(s) in the lasing material layer; and
(iii) forming a ridge from at least a portion of the second cladding layer.
Step (i) may be carried out by known growth techniques such as Molecular Beam Epitaxy (MBE) or Metal Organic Chemical Vapour Deposition (MOCVD).
Steps (ii) and (iii) may be interchanged, though it is preferred to carry out step (ii) then step (iii).
Preferably the passive region(s) may be formed by a Quantum Well Intermixing (QWI) technique which may preferably comprise generating vacancies in the passive region(s), or may alternatively comprise implanting or diffusing ions into the passive region(s), and annealing to create a compositionally disordered region(s) of the core layer, having a larger band-gap than the Quantum Well structure.
Preferably the QWI technique may be performed by generating impurity free vacancies, and more preferably may use a damage induced technique to achieve Quantum Well Intermixing. In a preferred implementation of such a technique, the method may include the steps of:
depositing by use of a diode sputterer and within a substantially Argon atmosphere a dielectric layer such as Silica (SiO2) on at least part of a surface of the semiconductor laser device material so as to introduce point structural defects at least into a portion of the material adjacent the dielectric layer;
optionally depositing by a non-sputtering technique such as Plasma Enhanced Chemical Vapour Deposition (PECVD) a further dielectric layer on at least another part of the surface of the material;
annealing the material thereby transferring Gallium from the material into the dielectric layer. Such a technique is described in co-pending application entitled xe2x80x9cMethod of Manufacturing Optical Devices and Related Improvementsxe2x80x9d also by the present Applicant, and having the same filing date as the present application, the content of which is incorporated herein by reference.
Preferably in step (ii) the passive region may be formed by QWI into the region(s) to create a compositionally disordered region of the lasing material having a larger band-gap than the Quantum Well structure.
Preferably step (iii) may be achieved by known etching techniques, e.g. dry or wet etching.
Preferably a length of the passive region is shorter than a length of the device. This arrangement provides a passive region adjacent a gain region.
More preferably, at least part of the passive region may be broader than the ridge. The passive region therefore provides a diffractive region adjacent the ridge which confines an optical beam within the QW structure. Advantageously the ridge may not extend over the said passive region.
Preferably the method may include the step of initially providing a substrate onto which is grown the first cladding layer, core layer, and second cladding layer, respectively.
Preferably, step (ii) may be performed by generating impurity free vacancies, and more preferably may use a damage enhanced technique to achieve Quantum Well Intermixing.
Preferably, the method may include the step of applying electrical contact layers to a surface of the lower cladding layer and a surface of the ridge. Alternatively and preferably, electrical contact layers may respectively be applied to a lower surface of the substrate, and an upper surface of the ridge.
In an embodiment of the invention, one of the electrical contact layers may be applied to a portion of the ridge so that the semiconductor laser device has an active region, and at least one second passive region below, ie within the area of the ridge. Preferably the portion of the ridge may be adjacent a first end of the device, such that the second passive region is provided at or near an xe2x80x9coutput endxe2x80x9d of the laser device.